The contact process is a method for manufacturing sulfuric acid in the high concentrations needed for industrial processes. Platinum was originally used as the catalyst for this reaction; however, because it is susceptible to reacting with arsenic impurities in the sulfur feedstock, vanadium(V) oxide (V<sub>2</sub>O<sub>5</sub>) has since been preferred.

History

This process was patented in 1831 by British vinegar merchant Peregrine Phillips. In addition to being a far more economical process for producing concentrated sulfuric acid than the previous lead chamber process, the contact process also produces sulfur trioxide and oleum.

In 1890 John Brown Francis Herreshoff developed a form of the contact catalytic process for the company of which he was a partner.

In 1901 Eugen de Haën patented the basic process involving combining sulfur dioxide and oxygen in the presence of vanadium oxides, producing sulfur trioxide which was easily absorbed into water, producing sulfuric acid. This process was improved remarkably by shrinking the particle size of the catalyst (e.g. ≤ 5000 microns), a process discovered by two chemists of BASF in 1914.<!--10.1007/978-3-642-93278-6_1 https://archive.org/details/in.ernet.dli.2015.4339/page/n3/mode/2up-->

Process

The process can be divided into four stages:

  1. Combining of sulfur and oxygen (O<sub>2</sub>) to form sulfur dioxide, then purify the sulfur dioxide in a purification unit
  2. Adding an excess of oxygen to sulfur dioxide in the presence of the catalyst vanadium pentoxide at 450&nbsp;°C and 1-2&nbsp;atm
  3. The sulfur trioxide formed is added to sulfuric acid which gives rise to oleum (disulfuric acid)
  4. The oleum is then added to water to form sulfuric acid which is very concentrated. Since this process is an exothermic reaction, the reaction temperature should be as low as possible.

Purification of the air and sulfur dioxide (SO<sub>2</sub>) is necessary to avoid catalyst poisoning (i.e. removing catalytic activities). The gas is then washed with water and dried with sulfuric acid.

To conserve energy, the mixture is heated by exhaust gases from the catalytic converter by heat exchangers.

Sulfur dioxide and dioxygen then react as follows:

:2 SO<sub>2(g)</sub> + O<sub>2(g)</sub> ⇌ 2 SO<sub>3(g)</sub> : ΔH = -197&nbsp;kJ·mol<sup>−1</sup>

thumb|Proposed mechanism for the oxidation of sulfur dioxide over vanadium oxide catalysts

According to Le Chatelier's principle, lower temperatures favor formation of sulfur trioxide, but if the temperature is too low the reaction rate becomes uneconomical. In industrial practice, the oxidation is therefore carried out at about 420 to 620&nbsp;°C over vanadium(V) oxide-based catalysts, typically around 450&nbsp;°C, at near-atmospheric pressure. Below this range the catalyst is deactivated by formation of vanadium(IV) compounds, while at higher temperatures it begins to decompose.